Electromagnetic clutches in the known art include the electromagnetic clutch disclosed in Japanese Unexamined Utility Model Publication No. H 2-2537. This electromagnetic clutch includes a hub having a flange and connected to a rotating shaft, an inner plate fixed onto the flange, an armature plate which is attracted to a rotor member so as to rotate together with the rotor member when a field coil is excited, an outer plate fixed onto the armature plate so as to constitute a part thereof and an annular rubber cushion member disposed between the inner plate and the outer plate and fixed onto the two plates.
To describe the electromagnetic clutch in further detail in reference to the drawings and the like included in the publication, the hub and the inner plate constituted as separate elements from each other are connected via a rivet. In addition, the surfaces of the outer plate and the inner plate facing toward the armature plate are aligned along the radial direction. While the portion of the surface of the rubber member facing toward the armature plate, which is on the outer plate side, is not in contact with the armature plate, the portion of the surface of the rubber member on the inner plate side facing toward the armature plate includes a projected part made to project further outward along the axial direction relative to the portion on the outer plate side and the projected part is in contact with the armature plate. The gap between the inner plate and the armature plate is eliminated by inserting a washer at the rivet having a measurement along the axial direction substantially equal to the measurement by which the projected part of the rubber cushion member on the inner plate side is made to project out and then by firmly holding the washer between the armature plate and the outer plate when locking the armature plate to the outer plate with the rivet.
The extent of the engagement noise that occurs when the armature plate is attracted to and contacts the rotor in an electromagnetic clutch needs to be minimized. Such engagement noise may be reduced by increasing the volume of the portion of the rubber cushion member on the inner plate side. However, if the volume is increased in the structure adopted in the electromagnetic clutch disclosed in the publication quoted above by, for instance, increasing the thickness of the rubber cushion member on the actuator side, the distance between the outer plate and the armature plate is bound to become more significant, which necessitates a larger number of washers to fill the gap and leads to a problem in that the electromagnetic clutch itself and ultimately the slave device such as a compressor cannot be provided as compact units.
In addition, while the extent of the engagement noise is likely to be reduced by fixing the portion of the rubber cushion member on the outer plate side in a compressed state onto the armature plate, the portion of the rubber cushion member on the outer plate side is not in contact with the armature plate in the electromagnetic clutch disclosed in the publication and thus, the structure achieved by fixing the rubber cushion member on the outer plate side in the compressed state onto the armature plate cannot be adopted in the electromagnetic clutch in the publication.
In order to lower the production cost of on-vehicle air-conditioning systems, electromagnetic clutches need to be manufactured at lower cost, which necessitates the functions of the various parts of the electromagnetic clutches to be scrutinized thoroughly to eliminate any nonessential parts.
Furthermore, the epoxy coating that is sprayed over the entire rubber cushion member to prevent corrosion normally coats the surface of the projected part of the rubber cushion member that comes in contact with the armature plate. In addition, a cationic coating is normally applied to the armature plate for rust prevention. If the epoxy coating is left on the contact surface of the projected part of the rubber cushion member which comes in contact with the armature plate, the projected part of the rubber cushion member becomes stuck to the surface of the armature plate coated with the cationic paint particularly under high-temperature, which may prevent the armature plate from becoming attracted to the rotor.
Accordingly, an object of the present invention is to provide an electromagnetic clutch with which the extent of the engagement noise occurring when the armature plate is attracted to the rotor is reduced, the number of required parts can be reduced and the armature plate can be attracted to the rotor with a higher degree of reliability.